In recent years, a secondary battery is widely used as a power source system in cooperation with a photovoltaic cell or a power generating device. The power generating device is driven by a natural energy such as a wind power or a hydraulic power, or an artificial power such as internal combustion engine. The power source system incorporated with the secondary battery is configured in such a manner that a surplus electric power is accumulated in the secondary battery, and an electric power is supplied from the secondary battery when a load device requires an electric power to thereby enhance the energy efficiency.
An example of the power source system is a photovoltaic power generating system. The photovoltaic power generating system is configured in such a manner that a secondary battery is charged with a surplus electric power, in the case where the amount of electric power to be generated by the sunlight is larger than the electric power consumption of a load device. Conversely, in the case where the amount of electric power to be generated by the sunlight is smaller than the electric power consumption of a load device, an electric power is outputted from the secondary battery to compensate for the shortage of electric power to drive the load device.
As described above, since a surplus electric power which has not been conventionally used can be accumulated in a secondary battery, the photovoltaic power generating system is advantageous in enhancing the energy efficiency, as compared with a conventional power source system which does not use a secondary battery.
In such a photovoltaic power generating system, if a secondary battery is in a full charge state, it is impossible to charge a surplus electric power, which causes electric power loss. In view of this, charge control is performed so that a state of charge (hereinafter, called as SOC) of a secondary battery does not reach 100% in order to efficiently charge the secondary battery with a surplus electric power. Charge control is also performed to keep the SOC from becoming zero so as to be able to drive a load device when needed. Specifically, charge control is normally performed in such a manner that the SOC is kept in the range from 20% to 80%.
A hybrid electric vehicle (HEV) loaded with an engine and a motor also utilizes the above principle. The HEV is configured in such a manner that a power generator is driven by a surplus output of the engine to charge a secondary battery, in the case where the output from the engine is large as compared with a power required for driving the HEV. The HEV is also configured in such a manner that the secondary battery is charged by using the motor as a power generator at the time of braking or decelerating the vehicle.
A load leveling power source or a plug-in hybrid vehicle effectively utilizing the nighttime electric power has also attracted attention in recent years. The load leveling power source is a system, wherein an electric power is stored in a secondary battery during the nighttime when the electric power consumption is small and the electricity rate is low, and the stored electric power is used during the daytime when the electric power consumption reaches its highest peak. The system is proposed to make the electric power generation amount constant by making the electric power consumption amount uniform, thereby contributing to efficient utilization of an electric power facility and reduction of the facility investment.
The plug-in hybrid vehicle is proposed to reduce the total CO2 emission, utilizing nighttime electric power. Specifically, the plug-in hybrid vehicle primarily performs EV driving of supplying an electric power from a secondary battery while driving an urban area where the gasoline mileage is low, and performs HEV driving of using both an engine and a motor during a long distance driving.
Incidentally, a secondary battery is degraded and the capacity thereof is reduced, as it is used. Accordingly, it is important to accurately determine the SOC of the secondary battery. For instance, if one fails to accurately determine the SOC of a secondary battery, and the secondary battery is overcharged, the long-term reliability such as the life of the secondary battery may be impaired. In view of this, it is necessary to precisely determine the SOC of a secondary battery in use, particularly, determine whether or not the secondary battery is in a near full charge state to perform charge control.
FIG. 7 is a graph showing a relation between an SOC and a terminal voltage of a secondary battery. In FIG. 7, the axis of abscissas denotes an SOC, and the axis of ordinate denotes a terminal voltage of a secondary battery in a non-load state, in other words, an open circuit voltage (OCV). As shown by the graph G1 in FIG. 7, a terminal voltage of a secondary battery is increased, as charging progresses and the SOC is increased, generally. Consequently, the SOC is detected by converting a terminal voltage of a secondary battery into the SOC, utilizing the characteristic as shown in the graph G1, conventionally.
However, as shown by the graph G2 in FIG. 7 for instance, some of the secondary batteries have a flat voltage characteristic that a change in the terminal voltage is small, as compared with a change in the SOC. In case of a secondary battery whose change in the terminal voltage is flat as compared with a change in the SOC au just described, the terminal voltage is moderately changed as compared with a change in the SOC. Accordingly, detecting the SOC based on the terminal voltage may result in lowering detection precision of the SOC. If charge control is performed based on the SOC having a low detection precision, there has been a problem that it is impossible to properly charge the secondary battery.
For instance, there may be a case that the SOC is erroneously determined to be 80%, despite that the SOC is actually 20%. In such a case, since the SOC is erroneously determined, charging is not performed, despite that discharging progresses and the SOC is becoming lowered. As a result, charging becomes insufficient, the dischargeable time is reduced, and the battery cannot deliver its performance fully. Conversely, there may be a case that the SOC is erroneously determined to be 20%, despite that the SOC is actually 80%. In such a case, extra charging is performed in excess of a full charge state, and there is a possibility that overcharge is performed. In that event, the life and the reliability of the battery may be impaired.
In order to solve the problems, there is proposed e.g. a method for easily detecting the SOC of a battery by using positive electrode materials of two or more kinds in a mixed state for enhancing detection precision of the SOC (see e.g. patent literature 1). The publication discloses that having two or more kinds of quasi-flat voltage portions of different voltage levels is advantageous in enhancing detection precision of the SOC. When there are a large number of quasi-flat voltage portions of different voltage levels, the gradient of a charging voltage is apparently large as compared with a change in the SOC in broad perspective.
However, in the technology disclosed in patent literature 1, since positive electrode materials of two or more kinds are used to enhance detection precision of the SOC, a battery characteristic is varied as compared with a secondary battery using a positive electrode material of one kind. As a result, it may be impossible or difficult to obtain an intended battery characteristic.
Patent Literature 1
    JP-A-2007-250299